Plotting and sewing a spinnaker

With my spinnaker model complete in Blender 3D and the paper cut plug-in generating 2D panels, I'm ready to go. Here's an update on the progress since the last post.

How do I mark out large cloth panels? I considered a number of possible approaches:

• Generate a series of vertex coordinates and manually plot them, then and join the dots;
• Create a scale drawing and linearly extrapolate each vertices then joint the dots;
• Project the image onto a large board at full size and plot;
• Build, borrow or buy a giant plotter;

Of the possibilities the manual approaches all seem too laborious, buying a second hand plotter is not a bad idea but they are huge and might cost a bit too much.

If the projector approach can work, it seems the best approach, but there are a set of possible problems:

Size: The digital projector in my office is fixed to the ceiling and the screen is about 2m (6ft) by 1.2m (4ft). I need an image about 3.5m by 1.5m for cloth and largest panel. The room is too small. I need to move the projector further away and need a bigger wall or high ceiling with clear floor space.

Accuracy: How do I know a 1m square projected onto the wall will be the same size wherever it occurs in the image (i.e. vertical and horizontal linearity)?

Marking out: How can I mark the cloth according to the projected image? The edges will be long sweeping curve.

Cloth Use: How do I know that I can fit all the panels on the size of cloth I've purchased, especially if I want the warp of the cloth generally aligned with the panel lengths?

Scaling: How do I ensure the shape projected onto the cloth is the correct size?

Below are the detail of what I finally did.

Plotting and sewing a spinnaker

I set up a projector in my office where I screwed a 2.4m x 1.25m MDF panel to the wall. I marked out the wooden panel with a 500mm grid. I set up the PC with Adobe Illustrator and created the test grid. I scaled the drawing until the projected grid was close to the marked out grid on the wall. I then fine tuned the projector position until the grid was aligned. To my delight I found I could achieve a gird which aligned to within a few millimeters over the range both vertically and horizontally.

Next I imported the 2D panels into Illustrator and set about moving the panels into position. Illustrator allows a maximum drawing size of 5.75m X 5.75m. Some of my cloth is over 5m long. It won't fit, so I worked at half scale.

I set up illustrator with a page size of 5.5m by 0.75m. The next problem is to ensure the imported Blender drawing is the correct size. I had anticipated this, so when I created the sail panels I also create some simple 1m by 1m squares as references. Once the panels were imported, I scaled everything until the reference square was 0.5m X 0.5m in Illustrator. When projected onto the wall, I checked the reference square size on the wall, and it was spot on at 1m x 1m.

The panels were still much longer than board. You can see one of the smallest panels in the picture below.

The procedure for larger is as follows:

• Project one end of the layouts onto the cloth. Mark the centres of the reference grid on the cloth as registration marks.
• Use an awl pushed into the board to press the flexible ruler (I used 2.5m long UPVC angle bead) against at two points on the panel. This gives a nice smooth curve which follows the projected edge. Mark out the panel with a soft pencil.

In the picture you can see the panel, the awl and some red lines which were added to the Adobe file to show where the seams go. A seam is where an extra 8mm is added to overlap the next panel. This is taped and then stitched. The procedure is:

• Stick a bit of masking tape to the panel and write on the panel numbers and mark the seams and the numbers of the adjacent panels. This will help with cutting and sewing later.
• Once the panels are marked, shift the cloth along and move the image left/right to get the remainder of the panel onto the board. (This might require a few steps to complete the whole thing).
• Carefully align the registration marks on the cloth with the projected image reference grid. 
• Continue marking out the panels.
• Shift the cloth up/down to complete the process for the whole vertical extent of the cloth.

With the cloth marked out, the question is how to accurately cut it out. Nylon should be cut using a hot knife to seal the edges and avoid fraying. I experimented with a temperature controlled soldering iron turned up high. The pointed tip worked well and easily cut the cloth, but a guide edge was required.

The cloth is first stuck together using basting tape. This double sided tape is a very thin acrylic tape with a paper backing. I found the soldering iron will run along the backed tape and create a good clean cut. I used the tape for all edges, ensuring I cut the inner edge where the panel has no seam, and the outer edge where the panel has a seam.

You can see a panel below, where the tape has been set out along the pencil lines and the cut made with the soldering iron. Note that the iron doesn't really burn the MDF board.

Here's another set of panels, taped out ready for cutting. Note the green tape labels that indicate the panel numbers and which edges are seams.

Here are some of the finished panels all ready to be stitched and a printed out plan with panel numbers for reference.

The panels are now assembled. I did this in three large triangular sections; the whole head section, the clew and tack sections with the natural vertical break. These were then to be sewn together. When assembling a section, I taped a panel and then stitched, then added another and stitched. This way only one panel, the new one, has to pass under the machine's arm as shown below.

I'm used a 3 step zig-zag machine which gives me a 6mm wide zig-zag. This is smaller than a pro-built sail, but I can't rip the seam apart so I think it is strong enough. Stitching over the tape is not a problem and fears of the needle gumming up have come to nothing. I'm using a fine point needle and V46 bonded polyster thread. Here are the finished seams:

You can see how the clew corner has not quite come together here. More on that later. Here's the finished assembly. Just the luff, leach and foot seams and corners to do.

Part Spun Fun With Sail Panels

Here's an update on the progress since the last post on designing the spinnaker.

Creating appropriate 2D panels

After some thinking I realised the way the 2D panels are created is important. The model I initially created in Blender was a "perfect" smooth curved surface (a nurb surface). This was based on a real sail design the I borrowed! and scaled a little to the boat.


To make 2D panels, I first needed to create a mesh representation from the perfect surface. But it occurred to me that the type of mesh was important. Each panel in the sail should be something which can be folded out flat without any stress in the panel. This is not the case if the initial mesh shown below is used.

The problem is the software simply cannot make a flat 2D panel using the mesh made of the back grid cut along the red seams. It can only approximate this. i.e. in the resulting panels the mesh edges lengths will not be the same when flattened as they were the curved model. i.e. the panel must be stretched a little to make it flat. This results in errors in the panel 2D shapes.

To solve the problem, I first converted the perfect sail into a near perfect fine mesh. Then I cut the sail into the panels. I then took each panel and deleted the internal mesh leaving only the perimeter vertexes for a panel as show below.

It should be understood that the edges of the panel (hole) are not straight, so the cut out is not flat (see below). I then created a new mesh of triangles using the perimeter vertices. This gives a mesh that can be laid out flat without any stress. i.e. it could be made from paper panels.

This might seem a bit complicated or over the top. Perhaps it is? However, I arrived here after lots of experimentation and only this way can I create 2D panels that when sewn in cloth and flown from the boat, should, in theory, adopt a shape close to the model shape!

Here's a single panel that can be flattened out, shown in its original position within the sail.

So how to mark out 3.5 meter long panels from the computer models ?